240 VAC Motor on 480 VAC VFD

[Barry1961]

I had a call from a customer who was using one of the new Allen Bradley VFD's running two 1/10 hp, 240 VAC, 3 Ph, gearmotors. The VFD had 480 VAC, 3 Ph power but was reducing it to 240 VAC for the motors. Both gearmotors were about 150' away from the VFD. There was not any filter between the VFD and motors since it was 240 VAC.

The motors were failing and drawing high amps. I told the customer that I suspected that the VFD was just reducing the pulse width and not the DC bus voltage so he would need to put filters between the motor and the VFD.

Was I right?

I thought about telling him to try running 1 ph and neutral for VFD power to get 277 VAC but had a bad feeling about it. Would this have worked? I have heard the new Allen Bradley,(PowerFlex?), is a lot better than the old one but have never used one so don't know much about it.


Barry1961

 

[fangas]

Hi Barry,

150' is kinda pushing it for the power cables. Filters would certainly help w/ reflected noise and make the drives happier but, they won't do a lot of good good for that high DC voltage.

I suspect that if the drives are laid into a 277 V supply, they will fault out due to low DC bus voltages. Unless this is adjustable with the drive parameters.

My advice would be to replace the motors w/ 480V ones or replace the drives with 240 V ones. And supply them via a small step-down transformer. Most 240 V drives, at the Hp you reference, will derive 3-phase from 1-phase and at those levels it won't take much of a transformer.

Ed

 

[Marke]

Hello Barry1961

This is not an uncommon practice and usually it does not cause problems provided that the insulation in the motors is high enough.

The drive will be operating by "reducing the pulse widths" so the insulation will be subjected to the higher transients.
The cables are long and this can cause problems for the drive, but it is also possible that there are standing waves being established in that length of cable that will increas the stress on the insulation of the motors.
I agree, the use of an output sinewave filter would probably overcome the problem.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[buzzp]

I agree with you. I am no drive guy, but I can't imagine them reducing the DC bus voltage as this would generate heat unnecessarily. It gets what it gets according to the supply voltage, I would think.
I guess your thinking there are more rise and fall times with reduced widths generating more trash. Makes sense to me.